1. Technical Field of the Invention
The present invention relates to integrated circuits and, more particularly, to systems and circuits for communicating between integrated circuits.
2. Background Art
FIG. 1 illustrates a system 10 of integrated circuits including integrated circuits IC0, IC1, . . . ICn. The integrated circuits are connected through a bus 14. Integrated circuit IC0 may be, but does not have to be, a controller to control IC1 . . . ICn. The various integrated circuits may be the same or IC0 may be different than IC1 . . . ICn. System 10 may be a system that is referred to as multi-drop system in which one or more of integrated circuits IC1 . . . ICn are selectively joined to bus 14. Merely as an example, system 10 may be a memory system in which controller IC0 is a memory controller (either part of a processor chip or in a different chip from the processor) and in which integrated circuits IC1 . . . ICn are, for example, dynamic random access memory (DRAM) chips that are on one or more printed circuit boards (PCBs). They could also be in a multi-processor system.
A voltage source Vs in IC0 provides signals to bus 14. A termination resistor Rs is in IC0. This is referred to as series termination. Termination resistor Rs may be an actual resistor in IC0 or may be inherent in IC0.
Address, data, and control bits on bus 14 may be on separate conductors of bus 14 or they may be, for example, time division multiplexed or packetized. For example, bus 14 may include some conductors used to carry only address bits, some conductors used only to carry only control bits, and some conductors used to carry only data bits. Or, some conductors may be used to transmit some combination of address, control, and/or data bits at different times or through a packetized or arrangement.
FIG. 2 illustrates a system 20 including integrated circuits IC0, IC1, . . . ICn. The integrated circuits are connected through bus 24. IC0 may be a controller to control the other integrated circuits and can be the same as or different than IC0 . . . ICn. FIG. 2 includes a termination resistor Rt which is on a PCB, which may be the PCB to which IC1 . . . ICn are connected or may be on another PCB. The termination arrangement of FIG. 2 is referred to as parallel termination.
FIG. 3 illustrates a system 30 including integrated circuits IC0, IC1, . . . ICn. The integrated circuits are connected through bus 34. IC0 may be a controller to control the other integrated circuits and can be the same as or different than IC. . . . ICn. FIG. 3 includes a termination resistor Rs in IC0 and a termination resistor Rt which is on a PCB, which may be the PCB to which IC1 . . . ICn are connected or may be on another PCB. The termination arrangement of FIG. 3 is referred to as series parallel termination.
Bi-directional signaling refers to using the same conductors to transmit signals in both directions. For example, data may be transmitted either to or from an integrated circuit. If the same conductor is used in both directions, the signaling is bi-directional. The bi-directional signaling may be sequential or simultaneous. In the case of sequential bi-directional signaling, enable signals may be used to, for example, tri-state or turnoff unused drivers or receivers. In the case of simultaneous bi-directional signaling, the threshold voltage of the receiver may be changed depending on the state of the adjacent driver. For example, if the adjacent driver is transmitting a 0, the receiver threshold may be set to Vcc/4. If the remote driver is also 0, the threshold will not be met. If the remote driver is a 1, the Vcc/4 threshold will be met. If the I&t adjacent driver is also transmitting a 1, the receiver threshold may be set to 3Vcc/4. If the remote driver transmits a 1, the threshold will be met and if it transmits a 0, the threshold will not be met.